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Carbon Comp resistance varies with the voltage across the resistor. The resistance is different if you put 100V across the resistance than if it's got 0V across it. So if you put a 50V DC level across a CC resistor and a 100V sine wave superimposed on that, the sine wave will be measurably distorted by the resistor itself. We have resistor distortion.

The distortion is pretty much pure second harmonic. In small amounts, you can't hear second harmonic as distortion, only a certain amount of "sweetening" or liquidity to the tone. That's what carbon comp resistor mojo really is - the resistors are distorting, but in a way our ears like.

They do no good and can create noise where the signal level is small and the following amplification is high i.e. the input stage. Also, cathode resistors are a poor use of CC. They typically only have a few volts across them, and they're often decoupled with a capacitor.

Use CC's where there's big signal - plate resistors, and ideally the stage just before the phase inverter. With plate resistors carrying the highest signal voltage in the amp, the phase inverter is ideal if it's not enclosed in a feedback loop.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hi guys,

I just finished an sIII build. I got to crank it this morning, and let me tell you my son did not find it as pleasing as I did. What the hell, he has to go to school anyway.

I was optimistic on what it would sound like, I am very partial to my 18watt TMB I built, and had a hard time believing I could ever stray from it. I built a TMB off the JA schematic, and it is a combo with two sammi 40 watt speakers. MY sIII is the head version, and I have it running through a 4x12 with celestion G12T-75's in it. I have plugged my TMB combo in it a few times, and it sounded ok. I was used to the sammi's. The sIII sounds like I just hit by a train standing in front of it. Really loud. I cannot compare the two just yet untill I plug the sIII into the sammi's. I'm sure it will deliver. One thing is for sure, THIS AMP IS QUIET!!!! No buzz, no squeal, and no hum. I thought my TMB was quiet. The TMB sounds like an FM radio not tuned to a channel next to the sIII.

The Build:
I picked up the parts from Stephen when I was in Toronto going to see Paul McCartney at the Air Canada Center. His wife let me in and fed me a beer. There is some great documentation with this amp. Anyway I had some nice reading material for the hotel. My wife didn't enjoy it as much as I did.

I got started right away when I got home. The layout was easy to follow. got all the parts in, the board, pots, switches etc... One thing I was a little dismayed about and I need to ask Stephen, with the impedance switch..... Do I need to use the 20A setup? Or will the 5A do?

Another issue I found was, the hole for the tranny has been moved to the side of the on/off switch. I use a different toggle switch for the power with a second set of contacts. I use it to install a bleeder resistor from B+ to ground when the power switch is off. IT drains B+ in about 10 seconds. My TMB I used 270K and it takes a good 45 seconds to bleed off B+. I use a 47k 5 watt resistor for the sIII.... works really well. So needless to say the switch is pretty bulky. I looked at it and said there is no way that switch will fit with the tranny in there. There is no clearance. Well.... it fit just fine, and the bleeder is working great.

So all the amp was in place and I had no trannies. I zipped off an e-mail to Steve over at S2amps, and voila.... I got his last set. They arrived three or four days later.... and thats across the border into Canada....Usually the parts hang around the border for at least that amount of time. Anyway.... I got the trannies yesterday, and I installed them. But wait..... I have no tubes..... I ripped the tubes out of my TMB and put them in the sIII. I think they will stay there for a while, at least untill I get a new set which will probably be next week.

The Power-up:
The moment has arrived.... Last nite at about midnight I was ready to power up. Its late, my head is spinning and my workbench is a mess. I did an initial power-up test. Powered up with no tubes in. everything looked fine. Voltages were where they were supposed to be. I said "Why not?" I stuck the tubes in and flicked the power switch. About 25 seconds later the I hit standby switch. To my dismay.... very low volume in the TMB and normal channel. I checked all the voltages, wrote them down, and zipped an e-mail off to Stephen after checking and double checking the wiring, layout and schematic. I was bummed. I was going to go to bed and tackle it in the morning. I just couldn't do that. I couldn't figure out what the problem was. I jacked up the normal channel volume with the guitar plugged in it. The volume was low, steady and the pot did nothing.
hmmm.... I turned it back down and plugged into the TMB channel. Same thing except their was little volume and the master and volume pot riased it a bit but not much.

I kept on probing with my meter trying to find some clue as to what was going on. I sent the voltages to Stephen.... He said they looked good.

With my guitar plugged into the TMB ch., for some reason I decided to turn up the normal channel. BAM... there's the volume for the TMB channel, with a loud buzz. I was puzzeled. Back to the schematic and poking with the meter... Well at least I have something to go on..

I decided to start with the layout and check wire by wire..... AH!! look!! I wired the TMB channel to the normal channel side of the the first 12ax7, and vica versa for the normal channel. DUMMY!!

As soon as I fixed that this thing came alive. I wanted to crank it bad last nite.

I played it low. for a while... nice sweet creamy clean tones. Much more so than my TMB. I think I am going to like this one...

This morning I jumped on it, and it is every bit as loud as my TMB, but it felt much nicer for some reason. The feel is a bit different. Maybe its the 4 G12T-75's... I don't know. I will find out a bit later today on a break, I am going to plug it into my combo.

Sorry for the lengthy post but, I had to say it. There's a new sherrif in town.... and his name is sIII!!! . I just have to build him a home now....

Bob

Pics will come lter today or tonight.... if your interested...

I find the sIII is much more stable than my TMB. I am now thinking of redoing my TMB board with the sIII layout. I got a chance to play more with the sIII.

Bob

Bob,

definitely interested in pics!! and CLIPS!!

so you're saying that the sIII is a better overall amp than the JATMB you made?

-Frank

Well.... suffice it to say, IT is way more stable. I attribute it to the layout I think. Don't get me wrong, I love my TMB combo, I just have these little things I don't like about it. To touchy, little things that bother me, like a bit of squeal sometimes, and noise. People say thats your wiring. Well, my sIII is a bit more messy than my JA TMB and it has 0 noise, even dimed, you can barely hear it. This is definately the long wires because of where the PI is... (slot 3). It is way more noisy than the sIII. I thought because the TMB was a type high gain amp, it is normal. When I powered up the sIII, I changed my mind quickly. In short I think the sIII is a newer better version of the TMB. I mean I had the thing dimed today, full tilt, and there was less noise than my TMB combo at 6 or 7.

I have a couple builds to do for friends, and no doubt what they will be.

The sound.... well..... pretty much what the JA TMB is but more tamed, but then again believe it or not, I think it has more edge than the JA version, even at lower gain. Could be the 4 Celestion G12T-75's I'm running it through though... To be fair, I have to connect the sIII in my combo with 2 12" sammi's.

I might play with some tweaks, but maybe I'll leave that up to my combo when I convert it.

I am working on installing a mosfet circuit with Mike to drop the voltage in the 18watt, so we can get that cranked el84 tone at lower volumes, so thats where my efforts are going to be probably starting next week. I have it slated to go into the sIII. I made provisions when I wired it last nite.

Other than that the sIII ROCKS!!!

You can see it on my workbench via my webcam.

http://www.rbevins.com/workshop

enjoy...

Bob

Thanks for the comments Bob...I really gotta step up my efforts....and start this thing...

So as far as tonality goes, the sIII and the TMB aren't that dissimilar?

-Frank

Hi Frank,

Well to be fair about it, I really should plug the sIII into the same cabinet the JA TMB is in. But I did have the JA TMB in the 4x12 cab only a few times, and yes for what I remember they are very similar, despite all the hype about the sozo's. I don't hear it. I think Stephen uses mallory's.

I will do that at lunch today and report back.

Well.... look what time it is.... lunch time for me...Time to play.

Bob

So.... here's the deal.. After been playing the sIII plugged into the TMB combo for about 30 minutes, It's a tough call. Too many variables.

In the sIII build I have S2 trannies and mallords, in the TMB combo I have Heybeor trannies and sozo's. Allthough very similar, there are some differences.

The sIII is definately brighter and a tighter low end. The JA TMB is a bit louder I think, but much darker. Trannies or caps?

The TMB controls on the sIII is more responsive. Allthough I like the normal channel on the JA TMB a bit better, I'll work on the sIII normal channel to get it the way I like.

Overall the winner I would have to say is the sIII.

If I had five amps to build, which I probably will shortly, I would build the sIII hands down, due to the fact it was much easier to build and to get right.

There is nothing wrong with either one other than the noise in the JA TMB. The differences are minor, if you call what I described above minor, the overall characteristic of these two amps are the same. Just a question of taste.

It bothers me that I cannot tell what is causing the difference... caps or iron, and I don't think I will be swapping trannies any time soon. I guess I will just live with it. IF thats my only problems, I don't think I am that bad off....

Bob

I'll bet you can make use of the tweaks in the schematic to get the sIII closer to the JATMB, if that was your intention.

I'm going to build an sIII, thats been decided for a while...but it's interesting to hear the direct comparison with another TMB...

-F

I will start with the normal channel to get it to where I want. The tweaks are for the TMB channel. I like the TMB channel on the sIII as it is, but I will have to play with it, just because I can....

Bob

Oh yeah, I forgot about them.... I will check them out.

Thanks

Bob, thanks for the input. Glad it all worked out.
I have lots of questons for you, but later.
However, what are you looking for in the Normal channel?
And, you HAVE to try jumpering the two channels!!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Bob, I was watching you fit the trannies on your voyer cam yesterday lol. I am getting really excited about this build, I have a stack of Sammi speakers to build into cabinets, what are you using in your 2 X 12?
Nigel

Hi guys,

Thanks Stephen.
I haven't tried jumpering the channels yet. I have an A/B/Y switch, and will be able to A/B the TMB and the sIII, when I get another set of tubes.

You can bet I will be doing that soon.

I want a more definitive sound in the normal channel, and less TIM distortion. there's a little too much for my taste. Maybe I'll try a Negative feedback on that stage.

Legin,

I have a 4x12 packed with Celestion G12t-75's in it. This thing brings that cab alive. One of my good friends was over tonight and he likes the sIII better than the JA TMB also.

My 2x12 has sammi speakers in it. The sammi's are really creamy, added with the sIII, together they really makes a nice sound. A little different from the JA TMB. I like it better but thats my taste. Someone else might have a different opinion.


I will beworking on a mosfet voltage dropper on the power section. I made some provisionos in the sIII chassis for that. I am having doubts on if I want to do it in that amp. maybe the TMB combo.

Anyway, sound clips are coming up shortly. I want to enter a clip into the RedHouse thread over at 18watt.com. I will use a combination of the sIII and the TMB combo.

Bob

The Normal channel in the sIII is the original Tremolo channel from the 18W, minus trem of course.

You can reduce the gain of the V1 for the normal channel by doubling R27. See how that sounds. There are other things as well, but try that first.

BTW, I would not add -ve feedback.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I have played with that resistor and C16 also. Allthough I haven't tried it on the sIII, but did with the tMB combo, and didn't like the changes. IT does decrease the gain, however I think the key is how it drives the PI.

What other changes do you have in mind?

How about something like this.

http://www.rbevins.com/18watt/nfbLocal.jpg

That doesn't work, it had the opposite effect, and there's a high pitched tone.

s2 & I put together some thoughts on how to tone tweak the sIII Normal channel.

The normal channel tone stack configuration is the original 18 Watt Tremolo Channel without the tremolo.

For less distortion:

The normal channel uses two capacitors in series (.0047 uF) off V1b 12AX7 plate with the junction of them going to a 470K bleeder (R29) then to ground . The bleeder resistor from the coupling cap to ground is R29. Replace it with a 100K or 56K.

Changing the cathode resistor by itself won't lower gain if it is bypassed by a capacitor (C16). If you lower the bypass capacitor you will filter out certain low frequencies, which will lower gain a tiny bit. Another option is to just remove the cathode bypass cap C16. Then maybe also increase the Cathode resistor R27 to 1k, 1.5K.

Bass Response:
To tighten up the bass (e.g. for humbuckers) , drop the cathode cap C16 from 47uF to something like 2uF, 1uF, or .68uF

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps